1. Field of the Invention
The present invention relates to a cam device for a photographic lens barrel.
2. Description of the Prior Art
Devices which move a lens optical system of an exchangeable lens along an optical axis to effect focusing or a magnification varying function are well known.
A magnification varying lens optical system and a compensation lens optical system are moved for a prescribed distance for effecting a magnification varying action, performing the magnification varying action and a compensation action simultaneously for effecting a zooming action. For control of the movement of each of the lens optical systems for this zooming operation an arrangement is employed such that a cam ring which is inserted and mounted on an inner or outer circumference of a fixed barrel of an exchangeable lens is provided and a guide groove having a spiral form is formed on the cam ring around an optical axis. At the same time, a straight forward groove in the direction of the optical axis is provided on the fixed barrel. Furthermore, a lens barrel to hold each of the lens optics is inserted into an inner circumference of the fixed barrel or the cam ring, and a pin is mounted on the lens barrel, the pin being arranged to extend through the guide groove and the straight forward groove. Each of the lens barrels is moved, by a straight forward operation or a rotating operation of an operation ring placed on an outermost position, by the amount of a shifting in the direction of the optical axis of the spiral guide groove of the cam ring, thereby varying the distance between the magnification varying lens optical system and the compensation lens optical system for effecting zooming.
A cam ring having a guide groove with a spiral form in the above-mentioned conventional exchange lens greatly affects the accuracy during zooming of the exchange lens and the cost thereof detracts from the possibility of mass production. That is, in a conventional exchangeable lens of a zoom lens assembly, the above-mentioned guide groove of the cam ring is formed by a cutting fabrication through the ring. Therefore, it is difficult to mass produce by machine fabrication, and it is necessary to produce a good finish with a fabricating precision in a plane of the groove of said guide groove. Thus, it is costly to secure a satisfactory preciseness, and hence it is difficult to reduce the cost or to enhance the precision.
Further, when the degree of inclination of the guide groove of a conventional cam ring is made large, the amount of shifting of each of the lens optical systems for an amount of movement of a zooming operation ring can be made larger thereby resulting in an overall dimension for the exchange lens which is shorter, thus enabling provision of a compact exchangeable lens with somewhat reduced weight.
However, if the degree of inclination of the guide groove is large, it adversely affects the physical strength of the cam ring. Therefore, the degree of the inclination of the guide groove relative to the optical axis must be limited to a prescribed range, and the dimension in the direction of the optical axis of the cam ring becomes longer. Thus, the distance through which the operation ring must move increases accordingly, and the length of the zoom lens assembly becomes longer and becomes inconvenient to carry.
Also a shifting mechanism having a conventional cam ring in a zoom lens involves problems in that a bearing or roller fixed on a pin mounted on an outer circumference of the lens barrel abuts on a plane of a guide groove of the cam ring. Therefore, when an impact force is imparted to a front plane of the exchange lens from outside or the lens is placed in a vertical position, an impacting force is exerted on the bearing or roller which is fitted in the groove of the cam ring. This force causes damage to the plane of the groove or generates uneven surfaces of the plane, thus causing an irregular feeling during zooming operation or lowering of the zooming accuracy.
As a method of solving the aforementioned problems in the cam ring of a zoom lens, there has been proposed forming the cam ring by a mold forming process of synthetic resin material as disclosed in U.S. Pat. No. 3,506,338.
However, in U.S. Pat. No. 3,506,338, one of two lens elements is pressed against a curvilinear shoulder 4 of a cam member by a spring member to effect control of the shifting movement along the curvature of the curvilinear shoulder. The system of U.S. Pat. No. 3,506,338 employs an arrangement wherein a spring 14 is biased between lens mounts to have a pin 13 pressed against a cam curvature thereby effecting control of shifting of two lens components 5, 6. This results in an increase in the number of component parts of the lens shifting mechanism and an increase in work during the assembly and the fabrication process which does not contribute to reducing cost.
The present application has previously proposed a structure for a lens barrel for the purpose of enhancing the optical and mechanical accuracy of the conventional lens barrel which is disclosed in U.S. application Ser. No. 166,825, now U.S. Pat. No. 4,386,829.
The lens barrel disclosed in said application as shown in FIG. 1 and FIG. 2 thereof comprises a tubular member 2 having a long linear guide groove 2a extending in a direction of the optical axis as shown in FIG. 1, and a mounting member not being shown in the drawing provided in the rear of the tubular member 2 for fixing the lens assembly on a camera. A second tubular member 3 supports a holding frame 4 of a first movable lens group L.sub.1 and is inserted into the inside of the tubular member 2.
A focusing operation ring 5 operates to shift the first movable lens group L.sub.1 forwardly and rearwardly along the optical axis, O--O', to effect a focusing operation. The focusing operation ring is integrally formed with the lens holding frame 4. A threaded part 4a formed at an outer circumference of the lens holding frame 4 is threadedly engaging with a threaded part 3a formed at an inner circumference of the second tubular member 3, and pulls out of the lens L.sub.1 by a rotating operation of the focusing operation ring 5 around the optical axis, to effect focusing.
A cam tube member 6 is placed around the outside of the tubular member 2 and is arranged to be rotatable around the optical axis on an outer circumference of the tubular member 2, and is prevented from shifting in the direction of the optical axis.
The cam tube member 6 has a projection part 6a having a rectangular cross section located at the hollow inner circumference of the member 6 as shown in FIG. 2, and one side of said projection part 6a is finished into a cam surface and controls movement of the first movable lens group L.sub.1 through a cam follower which is to be described later. The cam follower is placed between the cam tube member 6 and the second tubular member 3. The cam follower consists of a key member 7, a contacting piece 8 and a stop member 9, etc., which are inserted into the guide groove 2a as shown in FIG. 3 to FIG. 7 of the aforementioned application Ser. No. 166,825, wherein the contacting piece 8 is a roller.
The key member 7 and the roller 9 are fixed on the second tubular member 3 by a screw 10. The roller 8 is threadedly held in place by the screw 10 so as to be rotatable against the axis of the screw 10. One end of the key member 7 extends under the projection 6a of the cam part and further extends in a direction of the forward end of the lens, and the stop member 9 is placed on one end 7a of the thus extended key member 7 so as to go through the guide groove 2a.
The stop member 9 consists of a pin part 9a and a boss part 9b, and the boss part 9b is made to be insertable into a groove 7b of the key member 7, and the pin part 9a is pulled by a spring 11 placed within the groove 7b and presses the roller 8 against the cam surface.
A second lens holding frame 12 holds a second movable lens group L.sub.2 and is inserted into the tubular member 2. Movement of the second movable lens group L.sub.2 is controlled by a cam surface of a second projection (not being shown in the drawing) with a rectangular cross section provided at an inner circumference of the cam tube member 6, and the arrangement of its cam follower is exactly the same as that mentioned above in reference to FIG. 3 to FIG. 7, thus its explanation is omitted here.
A diaphragm device 13 is held in place by the second lens holding frame. A diaphragm aperture value setting ring 14 is inserted and mounted on the tubular member 2 and is associated with the diaphragm device to manually adjust the diaphragm aperture to a desired value. Since the arrangement of the diaphragm aperture setting ring 14 and the diaphragm device 13 may be of the conventionally known type, an explanation thereof is omitted.
The cam tube member 6 in the present invention can be made by a mold forming process of synthetic resin material (for example, polycarbonate).
When a strong impact is imparted to the lens assembly from a direction of an arrow shown in FIG. 1 of the present application to the front end of the lens, if a conventional cam follower is used the springs are made to expand by this force and the cam follower retreats from the cam surface, and the stopper pin then abuts on a protecting wall plane, where the force is stopped. When a plastics molded part is used in said cam, a pressure mark is generated at the protecting wall plane as shown in FIG. 3, and the pressure mark will become larger as the impact force becomes larger, and eventually the pressure mark will become so large as to effect normal zooming operation. The force is directed from a forward end of the lens toward the rear or mount side. However, when a force is applied in the reverse direction, since a gap t between the cam surface and the cam follower does not exist, there is no space for the spring force, and the undesirable effect will become greater. The resulting damage on the cam surface can be a critical defect for a zoom lens.